The Cave (2005)

Deep in the Romanian forest, a team of scientists stumbles upon the ruins of a 13th century Abbey. On further inspection, they make a startling discovery--the Abbey is built over the entrance to a giant underground cave system. Local biologists believe the cave could be home to an undiscovered eco-system, so they hire a group of American cave-explorers to help them investigate its depths. Jack and his brother Tyler are thrill-seeking professional cave explorers who run a team of the top divers in the world. They arrive in Romania with all the latest equipment, including a new type of scuba tank allowing a diver to remain submerged for up to 24 hours. The crack unit, which also includes Charlie and Buchanan, immediately begins their exploration. But what they find deep inside the cave is not just a new eco-system, but an entirely new species altogether.